Tape, such as magnetic tape, provides for physically storing data which may be archived or which may be stored in storage shelves of automated data storage libraries, and accessed when required. As an archival medium, tape often comprises the only copy of the data. In these and other situations, accuracy of the data and the prevention of damage to the tape can be a relatively high priority.
The servo system which moves the tape longitudinally is typically very precise, and the servo system bases the longitudinal movement on the instantaneous velocity of the tape. Tape drives frequently employ motors such as a DC motor, and a motor driver servo system for operating the DC motors, to produce well controlled motion parameters such as position, velocity, and tape tension. Precise control of tape velocity can facilitate correctly reading and/or writing data to the tape. For such control, a primary velocity signal is often generated, for example, employing a formatted servo track on the tape to directly measure the tape velocity. In the IBM LTO Ultrium Magnetic Tape Drive based on LTO (Linear Tape Open) technology, the servo track is made up of a sequence of repeated flux transitions, which produce a signal in a servo read head. The signal is a repeated set of bursts, that is peak detected to produce digital signals that can be used by logic to measure the time spacing between the bursts. The logic provides a count value of a reference oscillator to represent the time spacing of the bursts. This count value varies inversely with velocity, and is used to compute the velocity of the tape as it passes over the read head.
The primary velocity signal can be a very accurate, and direct, measurement of tape velocity. However, the primary velocity signal may not always be available for use in controlling the tape drive. During acceleration, deceleration, and while moving the read head between servo bands, the primary velocity signal typically is not available. There may also be exceptional conditions, such a loss of the servo signal, which make the primary velocity unavailable. Thus, it is useful if the tape drive is capable of controlling velocity without the aid of the primary velocity signal. During such times, an alternate, or secondary velocity signal may be utilized.
One example of a secondary velocity signal is derived from the back-EMF voltage in DC motors. As is known to those of skill in the art, the back-EMF voltage can be computed by subtracting the estimated winding resistance ohmic voltage from the motor voltage. The winding resistance ohmic voltage may be computed by multiplying the estimated motor current by the estimated winding resistance. The angular velocity of a DC motor may be estimated by dividing the back EMF voltage of the motor by the motor voltage constant of the motor. A servo system can estimate a longitudinal velocity VE of the tape by multiplying the calculated motor angular velocity ωC by the radius R of the tape at the reel the motor is driving.
Other examples of secondary velocity derivations may include use of encoders or analog tachometers. For example, it may be possible to estimate a suitable secondary velocity from the outputs of Hall sensors, depending upon the output resolution. For example, a DC motor may have 72 Hall states per revolution.
In tape drives, appropriate control of the tension on the tape can reduce excessive strain on the tape and thus reduce tape failure. In addition, reduction of tension variations can also restrict lateral motion and reduce timing induced difficulties in the data channel. Tension sensors can facilitate reduction of excessive tape strain. However, due to cost concerns, tape drives may not incorporate a tension sensor.
Precise control of tape velocity can reduce excessive tape strain. The secondary velocity is typically always available, but usually does not have the accuracy of the primary velocity signal. For example, in the case of the back-EMF measurement, the velocity signal typically includes error sources, such as motor torque constant tolerances, winding resistance tolerances, thermal effects, motor driver signal tolerances, and motor current estimation tolerances, among others.
These and other motor parameters and operational signals may be controlled to a degree through the manufacturing process by measuring and controlling the motor variations from the manufacturer, and measuring and controlling the variations in the assembled drive. In addition, motor parameters may be calibrated for each particular tape drive.
For example, U.S. Pat. No. 6,838,386 describes a calibration technique in which logic operates each DC motor of the tape drive under separate control at a steady state computed or estimated angular velocity ωC for at least one full revolution without driving tape. Thus, the motors may be operated when the tape cartridge is removed, for example.
Rotation index sensors may be used in this technique to indicate a full revolution of each of the DC motors such that the time duration of the full revolution of each of the DC motors may be measured. The actual angular velocity ωA of each of the DC motors may be determined by dividing 2π by the measured time of the full revolution of each of the DC motors. A calibration constant may be calculated for each of the DC motors by comparing the computed estimated velocity ωC during the full revolution, to the determined actual velocity ωA. When the tape drive is operated with a cartridge inserted into the tape drive, a longitudinal tape velocity for the tape may be estimated based upon the calibrated motor velocity of each motor.